In a normal metal, the electrons act relatively independently of both each other and the lattice of ions. Under certain conditions, however, the delocalised cloud can cause indirect bonding between electrons, thereby giving current enough coherence to resist any collisions or electrostatic interactions that individual electrons may experience. In this way, the only influence on the electrons is electron potential, a gradient of which is provided by an electromotive force. The result of this is a conductor without any resistance at all--a superconductor.

The temperature of the material is absolutely critical for electron bonding. The ion lattice must have such low energy that most of the valence electrons remain with their associated atoms, and the vibration of the lattice is only very slight. As a result, it is impossible to produce this state in temperatures above about 25K.
The boiling point of liquid helium is around 4K, so this is ideal for use with low-temperature superconductors.

Under these conditions, when an electron is liberated from an atom's orbital, the energy of the lattice is low enough that the moving electron exerts a significant attractive force on the surrounding ion lattice, leaving a region of low electron potential in its wake. This region then attracts other electrons causing them to follow the path of the first. As a result, an indirect attraction between the two electrons is formed, pairing them over a fixed distance--about a thousand times the spacing between adjacent ions in the lattice. This attraction is only possible if both electrons have opposed spins, which reduces the electrostatic repulsion between them. In fact, the two electrons in an orbital always have opposed spins and very similar energies, and
the valence orbital is destabilised under these extremely low temperatures. This means that most often the pairs will consist of electrons originating form the same orbital within a very short time of each other. This interaction between electrons is called Cooper Pairing, named for Leon Cooper who received the 1972 Nobel Prize for Physics along with John Bardeen and John Schreiffer.

Effect of Electrons on Metallic Ion Lattice
(Figure 6: Effect of Electrons on Metallic Ion Lattice) (Animated version. Not to scale)
Image created by the author for this website Taken from Superconductors.org

It is preferable for the electrons to remain in this state, separated by a distance much larger than would be the case if their energy levels dropped and they re-entered the orbital they had just left. The result is that virtually all of the delocalised cloud consists of paired electrons with energy only slightly higher than they would have in the ground state.

Not only constantly interacting with each other, but also being in the same quantum state gives the whole cloud of electrons a high degree of coherence despite every electron not being paired to every possible other. This means that even if one pair of electrons independently receive enough energy to break the Cooper pair, they will not dissociate with each other unless they also reach the next quantum energy level, bringing a far greater degree of stability to the superconductive state than might otherwise be expected.

However, the paired state is still relatively fragile, because the amount of energy required to raise the quantum state is fairly trivial. As a result, there are several considerations which ensure that a superconductor remains in this state.
• If the temperature exceeds a critical threshold, the electrons throughout the whole material will lose coherence due to increased kinetic energy. This will not only physically break the bonding within the Cooper pairs but also provide the ion lattice with enough energy to resist attraction to passing electrons, preventing broken pairs from reforming.
• The magnetic field applied must be below the critical field strength, or electrons caught in it may gain kinetic energy as a result of the motor effect. Because of this, superconductors are seldom used with alternating currents.
• Excessive charge density can lead to disruption of the lattice, causing it to be unable to mediate the interactions between pairs of electrons. This is the result of so many electrons being present that any difference in electron potential created is evened out more quickly than Cooper pairs can form. In addition, a very high current will generate a magnetic field strong enough to destroy the superconductivity.

As soon as the pairs are separated, the material reverts to being a conventional conductor once again.